Hill End to Spaniard Rocks & Back: Step-by-Step Part 5

The next stage of the walk from Hill End to Spaniard rocks saw an even greater reduction in the numbers of washed up starfish, and correspondingly greater concentrations of seashells in the strandlines, mostly empty shells of the bivalve Pharus legumen. The fine black detritus arranged itself in elaborate patterns mostly to do with the drainage of water back down the beach.

The sample of the fine dark particles that I took at the time, turns out to have a very interesting composition now I have had a chance to examine it under magnification back home. There is a fair proportion of small dark decaying wood fragments but most of the black material is composed of minute shiny hard  particles of coal (what you might call coal dust). The coal is not difficult to account for since coal mining and its export from nearby docks was a major industry in the past. There are many ways the coal could have been accidentally deposited in the sea. Together with the coal dust there are various seeds that I am not able to identify and, most surprisingly of all, what seem to be myriads of delicate fish bones. In fact, so many small threadlike rib bones that the dried sample seemed to have a fibrous texture. Amazing.

Rocks and Pools on Burry Holms

The fantastically sculptured Carboniferous limestone around the tidal island of Burry Holms, which lies at the northern end of Rhossili Beach on the Gower Peninsula in South Wales, provides a habitat for many intertidal species.

The exposed rocks between the highest and lowest tide levels are covered with a patchwork pattern of permanently attached dark mussels and pale acorn barnacles on which thousands of roaming dog whelks feed. Periwinkles and limpets graze on the algal films that cover the rocks and the shells. The curiously curving contours of the rocks supply numerous sheltered micro-habitats in the form of small hollows, crevices, gullies, overhangs, and pools.

Some of the pools are only just big enough to accommodate a couple of sea anemones and a few dog whelks. Some bigger pools are almost perfectly circular smooth basins dissolved into the stone, characteristically highlighted in summer by vivid green soft seaweeds concealing minute fish and multitudes of striped top shells and other gastropods. The occasional deeper pool  becomes a safe haven for clusters of common starfish and small shrimps; while wet overhangs and clefts display numerous beadlet sea anemones in a vast array of colours from pale khaki to bright red, together with rounded mounds of orange sponge.

All the organisms that live on the rocks in the inter-tidal zone contribute to the process by which the rocks are shaped. Frequently, this is done in a slow, subtle, and imperceptible way by the actions of epilithic and endolithic micro-organisms such as bacteria, fungi, algae, and lichens, and by the way these microscopic organisms are scraped from the surface and surface layers of the limestone by grazing seashore creatures.

Sometimes, the erosion is visible to the naked eye – as in the circular “home bases” that limpets have created by the continual grinding and wear of their shells against the rock as they settled in the same place each time after foraging trips; together with acid dissolution of the stone by their waste metabolic by-products. Another easily observable kind of bio-erosion damage is the burrowing activity of marine polychaete worms and boring bivalved molluscs. These small holes in rocks are often clustered in a band immediately above and below the water line of pools but also in any continually wet or damp grooves and channels. The overall persistent erosional activity of marine invertebrate organisms on intertidal seashore limestone over thousands and even millions of years contributes to the creation of fascinatingly sculptured karst topography like that seen around the island of Burry Holms.

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Tube feet on starfish & sea urchins

Tube feet on a starfish: Starfish (sea star), with extended tube feet and spiny pink skin, eating a mussel (1)

If you have a very close look at a living starfish or sea urchin, you can see that they have hundreds of very mobile, translucent, tube-like structures with suckers on the end. In starfish (or sea stars as they are called in the States) these tube feet or podia are found on the underside in the ambulacral groove that runs the length of each arm. In sea urchins they are found protruding from all surfaces from between the spines. In both these types of Echinoderm the tube feet are used to capture food particles and transfer them towards the mouth – which is on the underside of the creatures. In addition, in starfish, they are used in locomotion – for walking across surfaces.

The tube feet are operated in an ingenious manner that is unique to the echinoderms.  Echinoderms have what is called a water-vascular system providing the basis for podia movement. Water is taken into the animal, through a small special button-like structure known as a madreporite, to fill a system of canals, cavities, and valves lying within the body wall. The tube feet are extensions of this water vascular system. So the hollow tube feet are filled with fluid. There are longitudinal muscles in the podia walls; these are contracted and relaxed against the water pressure in order to extend, retract, and change the shape of the tube foot. The whole mechanism is like a hydraulic system. The flat-ended sucker tip of the tube foot also produces copious amounts of sticky secretion that helps it to adhere to surfaces.

[I took these photographs at the Hatfield Marine Science Center of Oregon State University, U.S.A.]

Tube feet on a sea urchin: Sea urchin with slender red tube feet extended between long purple spines

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Stranded starfish at Rhossili Bay

Starfish at Rhossili Bay: Stranded starfish near Burry Holms at Rhossili, Gower, South Wales, U.K. 15th August 2008 (1)

Thousands of starfish were stranded by high tides and stormy seas one August at Rhossili Bay on the  Gower Peninsula. These voiolent natural events happen from time to time in the ecology of the seashore. Although it is sad to see so many marine and seashore creatures perishing at once, it is all part of the bigger picture. These animals become part of the food chain for other creatures – the more obvious larger ones like crabs and birds and the smaller invertebrates like the sandhoppers.

These Common Starfish, Asterias rubens Linnaeus, on the sandy shore formed a virtual carpet of orange, pink, red, purple and every colour in between. Star shapes and contorted versions of them. Rough spikey textures on vividly coloured upper surfaces; and soft radiating rows of tube feet on paler lower surfaces. Spread across a wide strandline in an almost continuous mosaic pattern – interspersed with razor and other seashells and empty tests of sea urchins. The occasional crab feasting on the remains; raucous groups of big seabirds picking over the remains.

Common British starfish: Stranded starfish near Burry Holms at Rhossili, Gower, South Wales, U.K. 15th August 2008 (2)

Stranded Common Starfish (Asterias rubens Linnaeus) near Burry Holms at Rhossili, Gower, South Wales, U.K. 15th August 2008 (3)

Stranded Common Starfish (Asterias rubens Linnaeus) near Burry Holms at Rhossili, Gower, South Wales, U.K. 15th August 2008 (4)

Starfish washed ashore: Stranded Common Starfish (Asterias rubens Linnaeus) near Burry Holms at Rhossili, Gower, South Wales, U.K. 15th August 2008 (5)

Dead pink starfish: Stranded Common Starfish (Asterias rubens Linnaeus) near Burry Holms at Rhossili, Gower, South Wales, U.K. 15th August 2008 (6)

Crab eating starfish: Edible Crab (Cancer pagurus Linnaeus) eating dead stranded Common Starfish (Asterias rubens Linnaeus) near Burry Holms at Rhossili, Gower, South Wales, U.K. 15th August 2008 (7)

Upside-down stranded Common Starfish (Asteria rubens Linnaeus) still clinging to life near Burry Holms at Rhossili, Gower, South Wales, U.K. 15th August 2008, showing extended tube feet (8)

A multi-coloured carpet of stranded starfish (Asterias rubens Linnaeus) on the sandy strandline near Burry Holms at Rhossili, Gower, South Wales, U.K. 15th August 2008 (9)

Revision of a post first published 2 October 2009

COPYRIGHT JESSICA WINDER 2011

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Common Starfish from Rhossili Bay

Common Starfish, Asterias rubens Linneaus, on the causeway between Burry Holms and Spaniard Rocks at Rhossili Bay, Gower, South Wales, April 2009 (1)

One of many Common Starfish, Asterias rubens Linnaeus, cast ashore near Burry Holms on Rhossili Bay in April. This one was still alive but most of them perished. Starfish are Echinoderms and have five broad triangular arms which are between 3 to 5 times as long as the diameter of the central disc. This distinguishes them from the Brittle Stars which have arms up to 10 times as long as the central disc. The so-called arms are also called ‘fingers’ – hence the common name of Five-Fingers. The scientific term for the arms is rays; and the axes (or way they are arranged) are termed radii.

The arms are swollen, and have a warty-looking surface due to the presence of papulae. Common Starfish can grow up to 500 mm in diameter (I’ve never seen one that huge) and they can be any of a number of colours from orange, red, purple to yellowish or mixtures of these with a mottled appearance. They are common on all types of substrate and are often seen stranded on sandy shores.

Common Starfish, Asterias rubens Linnaeus, from Spaniard Rocks, Rhossili Bay, Gower, showing detail of the dorsal surface with madreporite (2)

The picture above gives a detail of the  upper surface of the starfish. This is the side away from the mouth and is called the aboral surface. There are hard spines in the skin and these can be various shapes and designs on different parts of the body and serve different functions. Most of the white structures in the picture are spines but the larger roundish thing (at about 6 o’clock on the central disc) is the madreporite which monitors and controls the intake of sea-water for the all-important hydraulic system.

Common starfish, Asterias rubens Linnaeus, from Spaniard Rocks, Rhossili Bay, Gower, showing ventral or under surface (3)

This picture shows the underside or adoral surface of the starfish. It looks very different from the top surface. There is a groove along each of the arms with rows of mobile tube feet operated by hydrostatic pressure; each tube foot ends in a circular sucker disc by which it can grip onto objects and ‘walk’ along. Spines line the outer edges of the grooves and all grooves lead to the mouth.

Common Starfish, Asterias rubens Linnaeus, from Spaniard Rocks, Rhossili Bay, Gower, showing detail of ventral surface with columnar spines and tube feet (4)

Starfish are related to Sea Urchins – both are Echinoderms with pentamerous symmetry (symmetry based on fives). Much smaller than the specimen above, and a different colour, the juvenile common starfish pictured below was seen near Spaniard Rocks on the same day as the starfish above. You can see how small it is by the marks on the pebble where acorn barnacles were once attached.

Tiny Common Starfish, Asterias rubens Linnaeus, near the Burry Holms causeway, Rhossili Bay, Gower, South Wales, April 2009 (5)

Revision of a post first published 17 May 2009

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Dancing Star – Brittle Star, that is!

Before he was rescued, this upside down brittle star performed a series of rapid manoeuvres, twisting his arms this way and that. He looked like he was a dancing star in Saturday Night Fever!

If you want to see what he looked like the right way up, click HERE for an earlier blog post.

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Starfish riding a Common Whelk

On New Year’s Eve a couple of years ago, winter storms had stirred up the seabed offshore, dredging up all the marine invertebrates like Razor Shells, Common Starfish, Brittle Stars, Spiny Cockles, Common Whelks, and much more. These were lifted up by the waves and tossed high on the shore with a lot of debris and empty shells in abundance. Some of the animals struggled to survive and others profited from the feeding opportunity provided by the mass stranding. All were mixed up together – the dead and the quick. Maybe the starfish were just searching for firm substrates but a lot seemed to home-in on the roaming and predatory whelks – attaching themselves firmly to the shells and seeming to hitch a ride on the large gastropods.

 

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